The most important aspect of selecting circuit protection devices for trucks, off-road equipment and other mobile equipment is a complete understanding of the system to be protected. Because the circuit protection design is usually one of the last considerations, and time is always at a premium, this aspect of electrical system design is usually rushed. Mechanical Products, a manufacturer of thermal circuit protection devices, recommends an easy seven-step process for effectively selecting your overcurrent protection solution.
1. Determine what is to be protected and why. i.e.-device(s), component(s), circuit(s).
2. Determine how damaging overcurrents and natural inrush currents and surges can be developed in those items listed above.
4. Calculate the magnitude and duration of the potential fault currents of Step 2 as they relate to those items listed in Step 1. Determine maximum voltage requirements of the protective devices you will use. It is also important to consider what kind of environmental elements the circuit protection device will be exposed to.
5. List the supplementary requirements for the protective device. What will it be connected to? i.e. auxiliary switch for an alarm circuit, lighted actuation, environmental considerations, electrical trip, relay trip, etc.
6. Determine the regulatory requirements. Depending on your application, you might be required to use circuit breakers, automotive fuses and other circuit protection devices that are SAE rated or UL rated.
7. Choose a circuit protection device that meets the requirements of your application.
To offer more details on step 1. What is to be protected and why, keep the dynamics of circuit protection in mind. In order to avoid nuisance trips attributable to start-up inrush and harmless surges with power systems, it is necessary to provide a margin of tolerance between the steady state current of the circuit and the rating of the protector. In general, the recommended margin for fuses is 25%; for circuit protectors, 15 to 20%. Additionally, there is a trip window or tolerance on the calibation of the protection device.
For precise circuit breakers, this tolerance is between 25% and 35%. This means that a circuit breaker will hold 100% and will trip between 100% and 125% to 135% within the hour. Based on this common industry specification as an example, a 10 amp rated protector can be expected to hold 10 amps or 100%. It can also be expected to trip at 12.5 or 13.5 amps within an hour. The expected trip point is governed by the Maximuim Ultimate Trip (MUT) specification. In this example, the Maximum Ultimate Trip is 125% to 135%, depending on the circuit breaker's specifications.
The most important considerations when matching potential fault protection to a circuit protector are the I2t factor and the fault trajectories. The I2t factor proects the potential for damage in a component - i.e. automotive wire, motors, power rectifiers, transformers, etc. Generally, this factor is a measure of what a device can absorb and still survive. The measure is a function of current in amps versus time in seconds.
The fault trajectory is simply a graphic representation of a fault and, if it is specified as a current in amps and time seconds, then both the I2t and fault trajectory can be put on the same graph. The point at which two lines cross on the graph represents the condition where circuit or component damage can be expected. For further clarification, the trip curve of the specified circuit breaker can be superimposed on this graph, giving a visual indication of the level of protection offered by the circuit breaker and its suitability for the application.
It is necessary to consider all of the aspects of the circuit protector that affect its published operating characteristics when considering its trip curve graph. Specifically, variations in performance can be encountered when factoring:
1. Position differences in magnetic circuit breakers
3. Potential for overreaction to inrush currents often encountered in magnetic breakers
4. Effect of voltage on the published trip characteristics of magnetic circuit breakers.
Having considered all these variables, the designer must also weigh the ancillary product considerations of a circuit protection, i.e.- allowances between product cost and performance, supplementary requirements, and regulatory approvals.